Effect of annealing treatment on oxalic acid-assisted electrodeposited CdS thin films for enhanced solar cell performance

CdS thin films have been successfully electrodeposited by introducing oxalic acid in the electrolytic solution as a novel complexing agent to prevent sulfide precipitation. The CdS films were grown on an FTO/glass substrate at − 0.890 V for 10 min and then annealed at 120 °C and 400 °C, respectively, in air. X-ray diffraction revealed that the CdS films have mixed hexagonal and cubic phases with (311) cubic-CdS preferred orientation. Scanning electron microscopy (SEM) results illustrated a transition from compact grains with more spherical precipitations on the surface at 120 °C to denser and homogeneous structure with a large crystallite size at 400 °C. The energy dispersive spectroscopy (EDS) revealed a decrease in the S content and an under-stoichiometric composition of CdS film at 400 °C. The band gap value decreased from 2.47 to 2.24 eV as the annealing temperature increased, while optimum transmittance was obtained at 120 °C. Mott–Schottky analysis revealed n-type conductivity for both samples where the flat band potential and donor density vary with the annealing temperature from − 0.99 to − 1.02 V and from 3.9 × 10²⁰ to 1.1 × 10²¹ cm⁻³, respectively. The electrochemical impedance studies affirmed that the electrochemical process is under kinetic control and demonstrated lower R_CT at 400 °C. PEC measurements showed enhancement in the V_OC and J_SC at 400 °C, indicating improved sensitivity and efficiency for photodetection. The slow decay of dark and photocurrent was attributed to defects and local potential fluctuations within the films. These findings highlight the effectiveness of using oxalic acid in the deposition process of CdS thin films making them suitable for solar cell applications.